1. Field of the Invention
This invention relates to an optical device, and more particularly to an optical device in which the projecting beam from beam projector means is projected to outside through a part area of an imaging optical system and, of the reflected beam reflected by the outside and entering the imaging optical system, the reflected beam passing through another area of the imaging optical system is received by photoelectric light-receiving means.
2. Description of the Prior Art
In the field of measuring the distance to an object or detecting the state of focus of an imaging optical system with respect to an object, there is a so-called active distance measuring device or focus detecting device in which beam projector means is provided on the device side and a beam is projected from this beam projector means toward the object so that by utilizing a variation in the position of incidence, on a predetermined surface, of the then reflected beam from the object, the distance to the object is detected or the state of focus of the imaging optical system with respect to the object is detected. As such an active distance measuring device or focus detecting device, a TTL type focus detecting system has been proposed by applicant in Japanese Patent application No. 64747/1978 (Japanese Laid-open Patent application No. 155832/1979, whose U.S. counterpart is U.S. Ser. No. 43,250) now U.S. Pat. No. 4,357,085. As an embodiment in this previous proposition, there is disclosed a device in which a beam projector and a photoelectric light receptor are disposed rearwardly of an imaging optical system adjustable along the optical axis thereof and, of the reflected beam projected from the beam projector through a part area of the imaging optical system and then reflected by the object and entering the imaging optical system, the center of gravity of the beam entering the photoelectric light receptor through another area of the imaging optical system is displaced on a plane containing the light-receiving surface of the photoelectric light receptor in accordance with adjustment of the imaging optical system, so that the center of gravity of this beam is detected by utilization of the output of the photoelectric light receptor, whereby the state of focus of the imaging optical system with respect to the object may be detected to enable in-focus and out-of-focus and in addition, directionality of defocus during out-of-focus, namely, near-focus or far-focus, to be discriminated depending on the construction of the photoelectric light receptor in particular. A similar device is also disclosed as an embodiment in U.S. Pat. No. 4,384,199.
In such a TTL type active focus detecting device, a beam projector and a photoelectric light receptor are disposed at positions conjugate with a predetermined position on the prearranged imaging plane of an imaging optical system and therefore, when light is projected from the beam projector onto an object through a part area of the imaging optical system, part of the light beam reflected by the inner surface of a lens constituting the imaging optical system may enter the photoelectric light receptor. That is, the light beam having entered the imaging optical system from the beam projector is projected onto the object through a part area of the imaging optical system while, on the other hand, part of the light beam may return by the inner surface reflection in the imaging optical system and in this case, the light emerges as a divergent beam where the surface of the lens constituting the imaging optical system is convex, and the light emerges as a condensed beam where the surface of such lens is concave. Such light beam may enter the photoelectric light-receiving surface and the light beam which is a condensed beam and which enters the vicinity of the photoelectric light-receiving surface is great in energy intensity and has an adverse effect.
On the other hand, the energy intensity of the signal beam which is projected onto the object and then reflected by the object surface and enters the photoelectric light-receiving surface through the imaging optical system decreases in inverse proportion to the square of the object distance and also decreases in proportion to the reflection factor of the object. Further, the energy density also decreases where the projected beam pattern on the photoelectric light-receiving surface is bad in imaged condition and blurred. In the above-described device, the condition in which the signal beam is very weak like this must also be detected and if detection of weak light is possible, detection of in-focus with respect even to an object at a long distance and of low reflection factor is possible.
Under such a situation, the beam emerging by virtue of the inner surface reflection in the imaging optical system provides ghost or flare with respect to the signal beam which is projected onto an object and then reflected by the surface of the object and enters the photoelectric light-receiving surface through a part area of the imaging optical system, and overlaps with such signal beam and can no longer be separated therefrom by an electrical or optical method.
On the other hand, it is apparent that such ghost or flare is smaller in their influence as the frequency of the inner surface reflection increases and as the condensing property on the photoelectric light-receiving surface is smaller. Also, even if the beam is a condensed beam, where it is condensed at a position remote from the photoelectric light-receiving surface, there is little or no influence. However, the actual imaging optical system is great in number of lenses forming it and it is very rare that there is no ghost or flare which satisfies the conditions for entering the photoelectric light-receiving surface. As regards the energy intensity of ghost or flare, it is about 0.5.times.10.sup.-13 W to about 1.times.10.sup.-3 W, when a beam of 1 mW is projected from a light-emitting device and if all of the light beam after five inner surface reflections enters the photoelectric light-receiving surface. Such energy intensity is within a detectable limit for a photosensor such as a conventional Si-PIN photodiode and is of course a detectable energy amount for a photosensor having an accumuation effect such as CCD. Accordingly, where such an element is employed as the light receptor, the influence of ghost or flare resulting from at least five inner surface reflections becomes significant. Such ghost or flare fluctuates for the movement of the imaging optical system resulting from focus adjustment thereof, whereby the distribution of quantity of light on the photoelectric light-receiving surface varies and therefore, this becomes a decisive impediment in a system for detecting movement of the center of gravity of a signal beam.
As a method for reducing such ghost or flare, there would occur to mind counter-measures such as reducing the reflection factor of each lens surface, designing an imaging optical system in which there is no beam which returns to the photoelectric light-receiving surface and is condensed, and selecting a predetermined conjugate point position to thereby install the light-emitting device and the photoelectric light-receiving surface at portions free of influence, but such counter-measures could not be a positive method for avoiding ghost or flare and have thus been great limitations in designing.